SWE 423: Multimedia Systems

Chapter 8: Optical Storage Media

Preview of Optical Storage

Media

• A myriad of Optical Technology:

– CD-DA (the basis of all other CD formats)

– CD-ROM

– CD-I

– DVI

– CD-XA

– MD (Mini Disks)

– CD-WO, CD-MO, WORM (Write Once Read

Many)

– DVD

Optical Media

• Data is read and stored using laser light

• Audio CD (CD-DA): compact disc for

storing digital audio

– 601 MB

– up to 76 minutes of playing time

• CD-ROM: storing computer data

– 650 MB

What is a CD ?

• Metal layer (usually aluminum) reflects light

from a tiny laser beam into a light sensitive

receiver

• To record data, a laser is used to burn specific

pattern into the surface

• The surface of the reflective layer alternate

between lands and pits. Lands are flat areas

(0s); pits are tiny bumps on the reflective layer

(1s)

• Spiral track up to 3 miles

120 mm

CD

Physical

Layers

Protective Lacquer Coating

Reflective Aluminum Layer

Polycarbonate Substrate

Land Pit Land Pit Land

CD Characteristics

• The most important advantage of a CD is

over magnetic storage media is that 1.66

data bits / µm can be stored resulting in a

storage density of 1,000,000 bits / mm2.

– i.e. 16000 tracks/inch as compared to the floppy

disk’s 96 tracks/inch.

• Another advantage is that magnetization can

decrease over time while optical storage is

not subject to such effects.

Video Discs and Other WORMs

• WORM: Write Once Read Many system

• LaserVision video discs were used for the

reproduction of motion picture and audio

data

– Data is stored in analog-coded format

– Excellent audio/video picture quality

– Has a diameter of ≈ 30cm

– Stores ≈ 2.6 Gbytes.

Video Discs and Other WORMs

• Video discs were originally called Video

Long Play when introduced in 1973 in the

Philips Technical Review

– Audio signal is mixed with frequency-

modulated motion pictures

– A zero-transition, i.e. a change between a pit

and a land, can occur at any time.

• Pit length is not quantized, hence it is time-

continuous (analog)

Video Discs and Other WORMs• Many different WORMS, with incompatible formats,

were introduced– Interactive Video Disc

• Operates at constant angular velocity (CAV)– describes the motion of a body rotating at a constant velocity because as it

rotates it moves through a constant angle per unit time.

– revolution per minute (rpm).

• On each side– Up to 36 minutes of audio and video data at 30 frames/sec

– 54,000 studio-quality images can be stored

– By 1992, many WORM systems were introduced with capacities 600 Mbytes to 8 Gbytes.

– Jukeboxes use multiple discs to increase the capacities to up to 20 Gbytes.

• Advantage of WORMs over rewriteable mass storage is security against alteration.

WORM’s Characteristics

• Media Overflow

– Refers to problems occurring when a WORM

disc is almost full

• Check if data to be stored can fit on the disc

• Determine whether data can be split into 2 discs and

at what point in time

WORM’s Characteristics

• Packaging

– Refers to problems arising from the fixed block

structure of WORMS

• E.g. if the block size is 2,048 bytes and only one

byte is written, 2,047 bytes are recorded with

“empty content”

WORM’s Characteristics

• Revision

– Refers to the problem of subsequently making

areas as invalid.

• E.g. document edits (deleted portions are marked

invalid).

CD-DA

• Compact Disc Digital Audio

– Developed by both Philips and Sony

– Information is stored based on:

• Length of pits is always a multiple of 0.3 µm.

• A change from pit to land or from land to pit

corresponds to the coding of a 1 in the data stream.

– Therefore, it is discrete time, discrete value storage

CD-DA

• Audio data rate:

(# quantization bits /sample) * (# channels) *

(sampling rate)

• SNR = 98 dB, compared to that of 50-60 dB for

LP records and cassette tapes.

• Capacity (storage of audio data only)

– The play time of a CD-DA is at least 74 minutes

Capacity = # minutes * Audio Data Rate (in bits/s)

CD-DA• Eight-to-Fourteen Modulation

– Since the resolution of the laser would not suffice to correctly read direct pit-land-pit-land...sequences, i.e. ................, it was agreed that at least two lands and two pits must occur consecutively.

– Since a phase-correct synchronization signal (clock) cannot be derived from long lands and pits, the maximum length of pits and lands was limited to ten consecutive zeros as channel bits.

– As a result, bits written on CD-D do not correspond directly to actual information.

– In addition, filler bits are needed to avoid situations where the minimum/maximum limits are exceeded.

CD-DA• Error Handling

– Usually a result of scratches or dirt (called burst error)

– Two levels of error handling

• 2-stage error correction based on Reed-Solomon Algorithm

– For every 24 audio bytes, two groups, four bytes each, of correction bytes are included.

» First group corrects single byte errors

» Second group corrects double byte errors

• Real consecutive data bytes are distributed over multiple frames

– A frame consists of 588 channel bits corresponding to 24 audio bytes

– Burst errors will only damage part of the data.

Characteristics of CD-DA

• For uncompressed audio, CD-DA is very insensitive to read errors

• All CD-DAs are identical in terms of digital technology (leading to compatibility)

– 8-14 modulation and Cross-Interleaved Reed-Solomon Code are always used.

• Achievable error rate is too high for general computer data

– Necessitated CD-ROM extension.

CD-ROM

• Compact Disc Read Only Memory

• Specified by Philips and Sony

• For general computer data as well as uncompressed audio data

• CD-ROM tracks are divided into audio and data types, each carrying only one type of data

– Data tracks are usually located at the beginning of the CD-ROM

CD-ROM

• Blocks

– Has similar properties to sectors of other media

and file systems.

– Consists of 2,352 bytes of CD-DA block

• Audio data: 2,336 bytes

• User data: 2,048 bytes

• Two CD-ROM Modes exist

– CD-ROM Mode 1

– CD-ROM Mode 2

CD-ROM

• CD-ROM Mode 1

– Stores computer [user] data

Capacity: Approximately 650 Mbytes for a

playing time of 74 minutes.

• CD-ROM Mode 2

– Stores other media (error correction is left out)

Sync

12

Header

4

User Data

2,048

EDC

4

Blanks

8

ECC

276

Sync

12

Header

4

User Data

2,336

CD-ROM

• Logical File Format

– Logical file format and directory structure are

missing from the Mode-1 specification.

– High Sierra standard served as the basis for ISO

9660 standard describing the format.

• Logical block size: a power of two of at least 512

bytes that may not exceed the size of the actual

block.

• Defacto maximum is .........

CD-ROM

• Extensions to ISO 9660

– Rockridge Extensions

• Suitable for Unix file system with long filenames,

links and access rights

– Joliet file system

• Microsoft’s adaptation to Windows 95/NT file

systems

– El Torito

• Allows PC systems to boot directly from a CD-

ROM.

CD-ROM Limitations

• A random access time of about a second to an

individual track is much slower than that of magnetic

disks for data ( < 6ms)

– This is ok for audio data

– It is due to

• Synchronization time (clock frequency must be in phase with the

CD signal) ≈ few ms.

• Rotation delay: due to Constant Linear Velocity (CLV) playback

[Rotation Velocity is 530 rps on the inside and only 200rps on the

outside (locating and reaching a sector)]. ≈ 300ms

• Seek time: Determining the right spiral track. ≈ 100ms

• Concurrent playback of mode 2 audio data and

retrieval of mode 1 data is not possible.

CD ROM Extensions

• CD-I

– Announced in 1986 by Philips and Sony

– Capable of concurrent media ouptut.

– Appropriate devices that use CD-I were

available commercially in 1991

– Disappeared entirely from the market in 1997.

• CD-I Ready

– Can be played on both CD-DA and CD-I

devices

CD ROM Extensions

• CD-ROM/XA

– Compact Disc Read Only Memory Extended Architecture

– Established by Philips, Sony and Microsoft

– Addresses concurrent output of multiple media: Blocks of different media can be stored on one track, unlike CD-DA or CD-ROM.

– Many features similar to that of CD-I

– Two forms• Form 1 mode 2: Better error correction for user data

• Form 2 mode 2: More capacity to store compressed media including audio and video

CD ROM Extensions

• CD Bridge Disc

– Can be played on CD-ROM/XA and CDI

devices

• Photo Compact Disc

– Developed by Kodak and Philips

– Example CD Bridge Disc for storing high

quality photos

– Allows users to write to the disc

CD ROM Extensions• DVI

– Digital Video Interactive

– Consists of• Compression and decompression algorithms

• Highly integrated, dedicated h/w components for [de]compression in real time

• User interface

• Fixed data format

– Therefore, emphasis on compression and decompression algorithms, not CD technology.

– Uses CD-ROM mode 1 in addition to ISO 9660 as a basis for audio/video support system interleaved fileformat.

– Uses interchange level 1.• Filenames are limited to 8-point-3 characters from a predefined

character set

CD ROM Extensions

• CDTV

– Commodore Dynamic Total Vision

– Uses CD-ROM mode 1 and ISO 9660

– Uses interchange level 2

• Filenames of up to 30 characters.

• None of DVI and CDTV is currently in

reasonable commercial use.

CD-R• A special write once CD-ROM (CD-WO)

– Has a pre-engraved track

– CD-R drive burns pits into the blank CD-ROM

• Multiple sessions

– All CD systems assume that a lead-in area precedes the

actual data and is followed by a lead-out area

• Lead in area contains a table of contents for correct positioning

– This would necessitate all data to be copied in one atomic

action, during which the cd is inaccessible.

– To solve the above problem, multiple sessions were allowed

• Specified Max: 99 sessions Achievable Max: 46 sessions

Lead in Information Lead out Lead in Information Lead out

CD-R

– Until 1992, available devices could read only

one session.

• One-session CD-R are called regular CD-R, rest re

called hybrid CD-R

• CD recoding

– Recorders operate at 8x the player data rate.

– To produce a CD-R, the data rate must be

sustained through the write procedure

• E.g., CD-R Data is first stored on a hard disk

CD-MO

• Compact Disc Magneto Optical

– Introduced 1988

– High storage capacity

– Can be written multiple times

– Based on the principle that at higher temperatures, a weak magnetic field is needed to polarize the dipoles in certain materials

• Pit: coded with a downwards facing magnetic north pole

• Land: opposite to pit.

– Changes in the polarization of the light upon application of laser illumination enables reading the CD.• Hence, incompatible with all other CD technologies

– Did not make it commercially

CD-RW

• Compact disc ReWriteable

– CD-E (erasable) during development

• Cannot read CD-RW discs on every CD player since the reflectivity is lower than that of a CD–DA or CD-R.

Type Reflectivity

CD-DA 70%

CD-R/CD-WO 65%

CD-RW 15-20%

DVD• Digital Video Disk (Digital Versatile Disk)

• Backward compatible with current CDs– Logical refinement of CD-ROM/CD-R/CD-RW technologies

• The disc can have 1 or 2 layers and one or two sides– SLSS DVD can hold 4.38 GB

– DLSS DVD can hold 7.95 GB

– SLDS DVD can hold 8.75 GB

– DLDS DVD can hold 15.9 GB

• High capacity is achieved thru– Smaller pits � + track density

– Larger data area

– More efficient coding of bits

– More efficient error correction

– Lower sector overhead

CD vs. DVDCD DVD

Media Diameter ≈ 120 mm 120 mm

Media Thickness ≈ 1.2 mm ≈ 1.2 mm

Track Pitch 1.6µm 0.74µm

Min Pit/Land Length 0.83 µm 0.4 µm

Data Layers 1 1 or 2

Sides 1 1 or 2

Capacity ≈ 650 MB 4.38, 7.95, 8.75, 15.9 GB

Video Data Rate ≈ 1.5 Mbits/s 1-10 Mbits/s

Video Compression std MPEG-1 MPEG-2

Video Capacity ≈ 1 hour ≈ 2 – 8 hours

Sound Tracks 2-channel MPEG 2-channel PCM

5.1-channel AC-3

Optional (up to 8 streams)

Subtitles Up to 32 languages

HD-DVD

• Standard Definition (SD) Video becomes

less acceptable for 36+ inches screen sizes.

– High Definition TV Images (HDTV) are rated

“good” for 60+ inches screen sizes.

• HD DVD satisfies the public demand for

high quality HDTV content arising from

increased availability of large screens at

affordable prices.

HD DVD

• HD DVD shares the 12cm diameter and

1.2mm thickness of the current generation

of DVD discs, yet is able to deliver eight

hours of High Definition video on a dual-

layer, single-sided disc.

• A double-sided HD DVD-R disc can hold

up to 30GBytes of data.

Laser Wavelength

Comparison

More Information

• HD DVD Forum (Check the references in

WebCT)